I've got a leather-wrapped, custom fiberglass sub enclosure that will fit in the 1-series (coupe and convertible).

I think I paid about $400 to have this built. It's just collecting dust in my office, so I'm giving it away for $50. It sounded incredible when I used it with my 10" JL W6 and 500/1 amp. There's nothing wrong with the enclosure. I just don't need it anymore since I sold my car.